1. Field of the Invention
This invention relates to an optical disc and, more particularly, to improvement of a protective film on the optical disc and of a magnetic head device employed with the optical disc.
The optical disc for recording and/or reproducing information by laser light radiation, the digital audio disc or compact disc and the optical video disc or laser disc, have become popular.
The optical disc, such as a digital audio disc, has a transparent substrate, a pattern of pits and lands corresponding to information signals, and a reflective film formed by a thin metal film, such as an Al film, and a protective film for separating the reflective film from moisture in air and oxygen, are formed in this order on the substrate. The protective film is a layer of UV curable resin which is usually applied uniformly on the entire surface of the reflective film by spin coating or roll coating and cured in situ.
Meanwhile, with the above-described optical disc, only the information signals formed by the manufacturer on the transparent substrate are read, while it is not possible for the user to write information signals directly on the optical disc. Recently, a a magneto-optical disc has been developed as an optical disc on which information signals can be recorded and erased repeatedly by the user, and presented to the market.
With the above-described magneto-optical disc, a magnetic thin film having an axis of easy magnetization in a direction perpendicular to the film surface and exhibiting large magneto-optical effects is employed. For recording, the magnetic thin film is partially raised in temperature, by radiating a laser light beam, to a temperature higher than the Curie temperature or a compensation temperature to extinguish coercivity at the heated area for aligning the direction of magnetization with that of an external recording magnetic field. For playback, a laser light is radiated on the thin magnetic film to read out the recorded information by taking advantage of the magnetic Kerr effect or the Faraday effect.
The thin magnetic film having properties required of the recording magnetic layer may be exemplified by a rare earth-transition metal amorphous thin film, such as a TbFeCo based amorphous thin film.
The magneto-optical disc has a recording section formed by stacking a reflective layer and a dielectric layer on the rare earth-transition metal amorphous thin film, and the protective film is formed on the recording section. The magneto-optical discs are of two types, that is of a single side type and a double side type in which two magneto-optical discs are bonded together with the protective films facing each other so that recording/playback may be made from both sides. The single side type magneto-optical disc is attracting attention as a standard item.
Meanwhile, since the rare earth-transition metal amorphous thin film, formed as a recording magnetic layer, is highly corrosive, a water-proofness higher than that demanded of an optical disc, such as a digital audio disc, is demanded of the protective film of the magneto-optical disc. Above all, for the single side type magneto-optical disc, the demand laid on its protective film is more severe because the protective film is exposed directly to atmosphere.
For this reason, attempts have been made for forming a UV curable resin layer of a larger thickness or a higher cross-linking degree on the recording section as a protective layer for a magneto-optical disc. However, if the UV curable resin layer of a larger film thickness or a higher cross-linking degree is formed on the recording section, the substrate tends to be warped due to curing contraction. Consequently, a UV curable resin layer having the same film thickness and cross-linking degree as that of the digital audio disc is formed as a protective film, for the magneto-optical disk and sufficient resistance to corrosion is not achieved.